The present invention relates to a charge pump circuit for low voltage source, more particularly to a charge pump circuit for use with one or more photovoltaic solar cells.
Photovoltaic solar cells (PVSC) includes a thin, single crystal semiconductor substrate that has a p-n junction. Photo-electric excitation occurs within the bulk of the semiconductor when exposed to incident electromagnetic radiation, i.e., light. The excitation generates electron-hole pairs. The minority carriers, i.e., electrons in the p-type region and holes in the n-type region, migrate towards and across the p-n junction, thereby creating a potential difference across the p-n junction, from which electric power may be obtained by appropriately placing electrodes.
A single PVSC or solar cell generally produces a low DC voltage, e.g., less than 1v, when exposed to light. That is, the solar cells made of silicon generates about 0.65 volt each, and the solar cells made of GaAs generates about 1 volt. However, most electrical circuits require more than 1 volt to operate, e.g., 3 volts or more or 5 volts or more. Also the voltage needed to charge batteries to energy storage requires 1.2 volts or more or 3.9 volts or more.
Therefore, a module having a plurality of solar cells is used to operate the electrical circuits or charge batteries. The solar cells in the module are connected in series to increase the voltage output. For example ten solar cells may be connected in series to produce an output of about 6.5 volts. The solar cells may also be connected in parallel to increase the current output. Many solar cell modules have solar cells arranged in a matrix to increase both the voltage and current outputs. That is, the modules have rows and columns of solar cells connected in series and parallel.
Such a module is made by “off chip” rewiring in external assembly method that increases the cost of a photovoltaic (PV) module. This type of assembly is performed in emerging economies which have relatively low wages. In addition, care must be taken in selecting the suitable cells for connecting them in series. If one cell is defective, or has a lower efficiency, it becomes the “weakest link in the chain”.
Alternatively, the solar cells may be connected monolithically in a single chip by manufacturing the chip using a Silicon On Insulator (SOI) technology. In this technology, each solar cell diode is isolated electrically from each other and then connected by a suitable metal layer during the semiconductor device fabrication process. However, the SOI technology is considerably more complicated and expensive than the conventional semiconductor process technology for producing a plurality of independent cells. The SOI technology is also more likely to produce defective chips, thereby lowering the yield.